1. Field of the Invention
The present invention relates to an image forming apparatus such as a digital copying machine that reads an image in the unit of dots to copy the image on a paper.
2. Description of the Related Art
In conventional image forming apparatus, such as a digital copying machine, an object to be copied (an original) is illuminated with an exposure lamp. Then, a reflected light is converted electrically into an analog image signal by a photoelectric converting unit such as a CCD (charge-coupled device) sensor or the like. This analog image signal is A/D (analog-to-digital)-converted into digital image data. Thereafter, such digital image data is processed in various processes, thereby an image being formed. There is a proper light amount of an exposure lamp in the process for forming an image. The light amount of exposure lamp is fluctuated due to an aging change of the exposure lamp or the like. When the light amount of exposure lamp is not proper or when the light amount is too large, i.e., too bright, an image signal is overflowed. When the light amount is too small, i.e., too dark, a resolution is lowered so that the quality of picture to be formed is deteriorated considerably. The light amount of the exposure lamp can be controlled by changing the magnitude of a voltage applied to the exposure lamp. That is, a reference voltage that determines a range of A/D conversion is set in an A/D converter. Therefore, an input analog image signal must be adjusted so as not to exceed such range. Generally, in a digital copying machine, the analog image signal is adjusted such that, each time the copying is carried out, a white reference plate is read out by sequentially increasing the light amount of exposure lamp and an optimum light amount is selected on the basis of the resultant read data. For example, the 8-bit A/D converter whose reference voltage is 0 V and 5 V divides 0 to 5 V by 256. Thus, when the analog image signal input to an amplifier is in a range of from 0 to 4 V, the light amount of the exposure lamp is increased so that the analog image signal becomes 5 V. Therefore, the analog image signal is adjusted to be suitable for the reference voltage.
However, when the analog image signal is adjusted such that, each time the copying is carried out, the white reference plate is read by sequentially increasing the light amount of exposure lamp and then the optimum light amounts selected, then the light amounts that can be set in every copying operation are all canceled and then the white reference plate is read. There is then the drawback that a standby time for starting the copying operation is unavoidably increased.